1. Field of the Invention
The present invention generally relates to microelectronic devices, and more particularly to such devices having features on the order of a quarter of a micron or less, and switching times on the order of a picosecond or less. In particular, the present invention concerns the construction of devices employing resonant tunneling.
2. Description of the Background Art
Improvements in the processing of conventional integrated circuits has permitted the fabrication of devices having features on the order of about a quarter of a micron. Without cryogenic cooling, further downsizing of conventional bipolar and field-effect transistors results in excessive temperature rises causing unacceptable degradation of performance. Although further improvements in processing and materials may lower the practical limits on the size of integrated circuit devices, it is anticipated that future integrated circuit development will require unconventional devices that employ different principles of operation.
One principle of operation under investigation for future integrated circuits is resonant tunneling. Resonant tunneling is the operating principle of the well-known resonant tunnel diode. Because tunneling occurs through a potential barrier having a very narrow width, the frequency response of a resonant tunneling device is not limited by the diffusion or transit time of charge carriers. Instead, the frequency response is limited by the circuit capacitance and impedance of the device. The circuit capacitance and device impedance both scale directly with the area of the circuits and devices on the integrated circuit substrate, permitting devices to be down-sized to about the width of the potential barrier.
Tunnel diodes, however, have not been particularly useful for integrated circuit applications, primarily because the tunnel diode is a two-terminal device, and the current-voltage characteristic of the tunnel diode is dictated rather rigidly by the properties of the semiconductor material used in constructing the tunnel diode.
More recently it has become known to use molecular beam epitaxy (MBE) to grow atomically thin layers of single crystal material on a semiconductor substrate to construct tunnel barriers permitting one to engineer a device having a desired current-voltage characteristic. In particular, one may easily select the width of the barrier to adjust the tunneling current, and one may construct an array of barriers in series to increase the "peak" and "valley" voltages of the current-voltage characteristic. This ability to engineer the physical structure of the tunneling barriers provides a high degree of design flexibility quite independent of the properties of the semiconductor material used for fabricating the resonant tunneling device.
Currently researchers have turned their attention to providing a resonant tunneling diode with third control terminal called the gate, resulting in a gated resonant tunneling diode (GRTD), which can also be referred to as a resonant tunneling transistor or triode. Experimental devices and proposed designs have been similar to conventional integrated circuit planar transistors. In these devices the base or channel of the transistor includes the tunneling barrier. But the manufacturing process is rather complicated, and it is difficult to align the gate with the base or channel.